1. Field of the Invention
The invention relates to a flexible circuit board which can be used in place of a wire harness in various electronic apparatuses.
2. Related Art
A flexible circuit board provides the following features because of its flexibility, etc. Namely, the flexible circuit board can be set with bending it so that the space in an apparatus is effectively used, thereby increasing the degree of freedom in design; wiring can be placed in a high density and even in a bent portion so as to attain excellent workability in wiring and assembly; and wiring can be placed also in a movable portion so that currents or signals are easily transmitted. As miniaturization of, for example, an audio or video apparatus advances, therefore, a flexible circuit board becomes frequently used in place of a wire harness in such an apparatus.
As shown in FIG. 1, in a flexible board 1000, paste patterns (circuit patterns) 1002 are screen-printed by using thermoset silver paste, copper paste, or the like on one face 1011 of a base film 1001 which is molded from an insulating material such as polyester, or polyimide, and the paste patterns 1002 are dried and cured at a predetermined temperature so as to adhere to the film.
Terminals of plural paste patterns 1002 are exposed in pattern exposed regions 1004 which serve as a terminal area, a chip mounting portion, etc. Resist 1003 is applied to the other portions of the patterns 1002 so as to insulate and protect the patterns. Electronic parts such as resistors and LEDs are mounted on the flexible board, and then the board is used in various electronic apparatuses. Since the base film is flexible, the flexible board can be used in a portion of an electronic apparatus which is bendable or has a curved face.
However, such a conventional art flexible board has problems in that the paste patterns 1002 have a high electric resistance.
In the flexible board, the base film is configured by a film of polyester or the like which is caused by heat to largely shrink or deform and which has a low melting point. Therefore, the circuit patterns must be formed at a low temperature. When thermoset paste is used, a thickness of some extent is required for promoting hardening and reaction. This causes the whole of the flexible board to be increased in thickness with the result that the flexible board cannot sufficiently exert its flexibility, thereby lowering the flexing properties.
When silver paste is used, particularly, there occurs a phenomenon that variation with time or the like of the paste causes migration in the pattern exposed regions 1004 so that adjacent patterns are short-circuited with each other. In order to prevent such a phenomenon from occurring, countermeasures such as that carbon is printed on each pattern in an exposed region or that carbon is mixed with the silver paste are taken. However, these countermeasures cannot sufficiently satisfy the request for reducing the pattern pitch.
Namely, when migration occurs, a short circuit (a phenomenon in which patterns contact with each other) may be caused. Even when such a short circuit is not caused, the insulation resistance between patterns is lowered so that the withstand voltage is lowered, with the result that another short circuit phenomenon (a phenomenon in which an electric discharge occurs) is easily produced at a low voltage.
In the conventional art, as a countermeasure for migration, known are a method in which patterns are covered by carbon printing, and that in which patterns are formed by silver-carbon mixed paste. According to the latter method, migration cannot be perfectly prevented from occurring because silver is exposed.
The former method is known as a countermeasure which can prevent migration to a certain extent from occurring. However, printing must be conducted so that carbon covers a pattern. In printing, therefore, deviation between a pattern and carbon may occur. In the case where the pattern pitch is large (a pitch of about 1 mm) and the pattern width can be set with room, there arises no serious problem. By contrast, in the case where the pattern pitch is small (a pitch of 0.5 mm or less) and the pattern width cannot be set with room, the possibility that print deviation causes adjacent patterns to contact with each other is increased.